The HER2 (c-erbB-2) proto-oncogene is over-expressed in approximately 30% of the >160,000/yr new cases of breast and ovarian cancer, and correlates inversely with prognosis. While anti-erbB-2 antibodies and immunoconjugates show much promise for breast cancer therapy, a fully human high-affinity anti-erbB2 antibody is currently not available. Phage display libraries of antibody variable region repertoires have opened a novel route for the isolation of recombinant human monoclonal antibodies. What is lacking is a reliable means for completing the affinity maturation process in vitro. To accomplish this we have developed a computer-assisted method, called Parsimonious Mutagenesis (PM), for scanning antibody CDR for mutations which improve affinity (Balint and Larrick, 1993). This method facilitates the construction of "scanning" libraries which can "probe" the surface of an antigen a few residues at a time with a wide selection of preselected amino acid side chains to search out and identify new high-affinity contacts. These libraries are made with special low- redundancy "doping" codons and nucleotide mixtures designed to maximize the abundance of combining sites with predetermined proportions of altered sites. In Phase I we will use the method to "mature" the affinity in vitro of an anti-erbB-2 antibody which was isolated from a "naive" human repertoire library. In Phase II, we will conduct pre-clinical evaluation of the highest affinity antibodies from Phase I.